Field of the Invention
The present invention relates to an image pickup apparatus having a waterproof function and being capable of taking a good image both in air and in water.
Description of the Related Art
In recent years, with increased popularity of diving, an increase in number of underwater photographers, and the like, there are increasing opportunities for underwater photography, and an underwater camera is required to have portability, operability, an image pickup area, optical characteristics, and the like comparable to those in photography on the ground (in the air).
In general, as the underwater camera or an amphibious camera, a camera housed in an underwater housing or a camera equipped with a waterproof mechanism is used. However, water or salt water has a refractive index and a dispersion that are different than those of air, for example, a refractive index with respect to a d-line of about 4/3 of that of air, and a dispersion of about 62 in Abbe number. Therefore, when an image pickup optical system sufficiently corrected for aberrations in the air is used in water, a change in refracting action at an interface at which water and the image pickup optical system are brought into contact with each other occurs.
As a result, optical imaging characteristics significantly deteriorate due to a change in photographing field angle and changes in aberrations. In particular, in a case where the interface at which water and the image pickup optical system are brought into contact with each other is a planar surface, a photographing field angle in water becomes significantly small with respect to a photographing field angle in the air, and photography with a wide field angle becomes difficult.
As means for realizing the wide field angle during the underwater photography, a method in which the interface is formed into a shape that is convex toward an object side and a curvature thereof is increased to allow light beams to enter concentrically to mitigate the refracting action at the interface has been known. However, when the curvature of the convex shape of the interface is increased too much, a difference between refractive powers in the air and in water at the interface becomes too large, and various aberrations, in particular, field curvature is increased. Therefore, it becomes difficult to reduce the field curvature while realizing the wide field angle. Moreover, water has dispersion as well as the refractive index, and large lateral chromatic aberration occurs at the interface.
As means for reducing the increase in field curvature while realizing the wide field angle during the underwater photography, in Japanese Patent Application Laid-Open No. 2004-325711, there is described an image pickup apparatus utilizing a method in which a dome-shaped pressure-resistant window and a lens unit having a positive refractive power, which is removably insertable into an optical path, are included on an object side of an image pickup optical system. Moreover, as means for suppressing the lateral chromatic aberration, which occurs during the underwater photography, in Japanese Patent Application Laid-Open No. 2004-252219, there is disclosed an image pickup apparatus utilizing a method in which a filter attachment having a diffraction optical surface is attached on an object side of an image pickup optical system.
In Japanese Patent Application Laid-Open No. 2004-325711, the dome-shaped pressure-resistant window and the removably attachable lens unit having the positive refractive power are included on the object side of the image pickup optical system to form an afocal system when in water, with the result that a wide photographing field angle is secured, and that the occurrence of the field curvature is reduced. In the image pickup apparatus described in Japanese Patent Application Laid-Open No. 2004-325711, there has been a tendency for the dome-shaped pressure-resistant window to be increased in size with respect to the image pickup optical system. Moreover, in Japanese Patent Application Laid-Open No. 2004-252219, the filter attachment having the diffraction optical surface is attached on the object side of the image pickup optical system to suppress the lateral chromatic aberration generated when in water. However, the diffraction optical surface is extremely difficult to fabricate, and it has been difficult to obtain high optical characteristics both in water and on the ground.
The underwater camera is required to have good portability and attachability/removability during the underwater photography, and to generate small field curvature and lateral chromatic aberration during the underwater photography. The underwater camera is also required to have good optical characteristics during both the underwater photography and the photography in the air, to be easy to switch between the underwater photography and the photography in the air, and the like.